Apparatus for free abrasive machining or lapping is commercially available, being sold by the Speedfam Corporation of Des Plaines, Ill. See also, for example, U.S. Pat. Nos. 4,270,314 and 4,519,168 and other patent references cited therein. Typically, such free abrasive machining apparatus includes a horizontally positioned rotary wheel which provides abrasive action to the work piece. A short metal tube (a "truing ring") is usually placed on the rotary wheel with the axis of the tube perpendicular to the wheel, and the work piece is placed within the tube. Some form of pressure is applied to the work piece from a piston or a weight, and the truing ring is retained in position so that the rotary wheel can rotate under it, in such a manner that the truing ring can rotate. This rotation takes place because portions of the rotary wheel which are radially nearer to the center of rotation are moving more slowly than the corresponding portions farther away from the center of rotation of the rotary wheel.
Abrasive is placed upon the rotary wheel in slurry form, to accomplish the free abrasive machining process.
Periodically, it is necessary to check the flatness of the rotary wheel which generally must remain within certain limits of desired flatness in order to obtain correspondingly flat work pieces from the abrasive process. Typically, the rotary wheel tends to become concave due to the continued abrasive action of operation. When it reaches a certain degree of concave curvature, typically only a very few thousandths of an inch, the rotary wheel shape must be corrected, since it is no longer capable of providing the desired flatness by abrasive action to the work pieces.
Additionally, a so-called "tracking condition" can occur, created in the rotary wheel by placing excessively high pressure on the work piece, or by a failure to provide enough abrasive slurry. This tracking condition is an irregularity in the uniform curvature of the almost flat surface of the rotary wheel. The tracking condition can show up very quickly as a deviation in the shape of the surface by 0.0005 inch (0.5 mil.) or the like. In the event that the tracking condition takes place, it must be promptly detected and dealt with by correction of the rotary wheel, since parts produced on such a damaged rotary wheel are likely to not meet desired specifications. Unfortunately, the naked eye cannot determine when the rotary wheel should be corrected.
Accordingly, a test for flatness of a rotary wheel is conventionally used in which all of the abrasive slurry is removed from the wheel, a time consuming job. Then, a precision straight edge is placed upon the wheel, and the user attempts to slide shims of known thickness under the straight edge to determine deviations from flatness in the rotary wheel, to search for the presence of tracking condition or to generally determine when the wheel has lost its flatness to such an extent that it should be corrected.
This flatness test requires about ten to twenty minutes of time, counting the necessary cleanup of the wheel before the test can be performed. Also a considerable amount of fumbling usually takes place, as one searches for the right shim to slide under the straight edge, to determine the deviation from flatness of the rotary wheel.
In accordance with this invention, a measuring bar is provided which can be used without cleanup or removal of abrasive from the rotary wheel. This alone provides extremely significant savings of time on the order of ten minutes or more per measurement- Additionally, no shims are required, but instead a direct readout can be obtained from the measuring bar of this invention, so that one can determine the condition of the rotary wheel in the middle of a production run, without cleaning the abrasive off of its face, in a matter of a minute or so, rather than being saddled with a ten to twenty minute process, during which time production is halted.